1. Field of the Invention
This invention relates to a method of treating permeable formations using polymer gels, and in particular, to a method of treating subterranean formations in oil and gas wells using polymer gels to reduce the formation permeability, and more particularly to a method of treating such formations to retard or halt the unwanted flow of certain fluids within the formation.
2. Brief Description of the Prior Art
In oil and gas wells, it is sometimes desirable to block or reduce the permeability of certain zones or regions of the subterranean formation surrounding the well. For example, water from water-bearing zones can interfere with oil or gas recovery operations. The production of water from these zones may reduce the amount of hydrocarbons that can be recovered from the well and may require additional separation and disposal procedures if the water is removed from the well along with the oil and gas. Thus, it is beneficial to limit the flow from these water-bearing regions so that the desired oil or gas can be recovered more effectively.
Polymer gels formed from hydrophilic polymers, such as polyacrylamide, have been used in the past to reduce the permeability of selected zones within subterranean formations in oil and gas wells. These polymers are usually mixed with water at the surface to produce aqueous solutions which are pumped into the formation. A crosslinking agent is typically added to the solution at the surface, along with any delaying agents or gel inhibitors, so that once the solutions are in place within the desired region of the formation, the polymers begin to gel or crosslink so that the polymers gel in place and plug the pore channels and flow paths of the formation and the permeability of the formation is effectively reduced or blocked, especially in high water cut zones.
High-molecular-weight polymer solutions are characterized by a high pre-gel viscosity. For formations having a high permeability, i.e. greater than 1000 mDarcy, such as naturally fractured or operationally fractured, and formations which have a high fluid loss (high permeability) which may occur as lost circulation during drilling operations, high-molecular-weight polymers are often employed, because of their high pre-gel viscosity. High-molecular-weight polymer solutions produce less viscous gels which are less resistant to removal by formation fluids. In low permeability formations, low-molecular-weight polymers or non-ionic polymers may be used because of their lower pre-gel viscosity, which allows them to be more readily injected into the pore channels and smaller flow paths of the formation matrix. Larger quantities of the low-molecular-weight polymers are required to achieve higher viscosities. The low-molecular-weight polymers produce a highly viscous, solid-like gel, sometimes referred to as a "ringing gel". By comparison, the high-molecular-weight polymers produce less viscous gels, sometimes descriptively referred to as a "tonguing gel", or in some cases a more viscous "rigid gel", both of which are less viscous than the solid-like ringing gel produced by the low-molecular-weight polymers described above. A gel strength code is disclosed in U.S. Pat. No. 4,770,245 "Rate-Controlled Polymer Gelation Process for Oil Recovery Operations", issued Sep. 13, 1988 to Robert D. Sydansk and assigned on its face to Marathon Oil Company. This patent is hereby incorporated in its entirety by reference. Typically in formation treatments, a higher concentration of the low-molecular-weight polymer is required to obtain the desired final gel viscosity. For example, aqueous solutions of low-molecular weight polymers used in treating permeable formations may employ from 4 to 6 weight percent of polymer in the solution to provide an adequate final gel viscosity. By contrast, formation treatments utilizing high-molecular-weight polymers may contain, for example, only 0.045 to 1.5 weight percent of polymer and are less expensive. Because of the large amounts of the low-molecular-weight polymer necessary to produce the desired viscosity for most formation treatments, solutions utilizing the low-molecular-weight gels are more expensive and therefore add to the overall expense of the treatment.
What is therefore needed is a polymer gel and method of treating a subterranean formation with an aqueous solution of water-soluble, crosslinkable polymers which utilize lower concentrations of low-molecular-weight polymers while providing a suitable pre-gelled viscosity in the formation and a gel that is not easily flowed through, leached or removed once the gel is in place in the formation.